The present invention is related generally to data communication systems and, in particular, to free-space optical data communication networks.
Traditional telecommunication systems that connect two or more sites with physical wire or cable are generally limited to relatively low-speed, low-capacity applications. In order to address these limitations, recently developed systems utilize optical fibers. Yet, fibers still require a physical cable connection. To remove this limitation, systems utilizing the free-space transmission of one or more light beams modulated with data have been developed. Systems using such beams greatly improve data speed and capacity rates, up to 10 Gbps, over traditional wire-based systems and, at the same time, avoid the traditional communication system infrastructure cost of laying fiber cable to physically connect one site in the system to another. Instead of cables, free-space optical communications systems consist, in part, of at least one transmit telescope and at least one receive telescope for sending and receiving information, respectfully, between two or more communications sites. Each of these telescopes contains optics, consisting of at least a primary mirror and a secondary mirror or a lens. The transmit telescope uses its optics to transmit the light beam to the receive telescope. The receive telescope uses its optics to focus the incoming light beam onto the focal plane of the telescope. Generally, each telescope is attached to a communications network or other source/destination of information. In operation, the transmit telescope receives information from its respective network via cable or wireless transmission, and then transmits a light beam modulated with this information to one or more destination receive telescopes. Each receive telescope then relays data to its intended destination in its respective network via a cable or wireless transmission.
The aforementioned free-space communications systems would, therefore, appear to have the benefits of reducing costs associated with installing and maintaining physical hard-wired portions of networks while, at the same time, increasing transmission capacity. However, free-space optical communications may be hampered by a variety of factors. For example, since the transmit and receive telescopes may be located a great distance from each other, initial alignment of the telescopes, to insure that the transmitted light beam is incident upon the focal plane of the receive telescope, may be difficult to achieve. Additionally, even if initially aligned, misalignment of the transmit and receive telescopes may result from any displacement of the light beam during transmission or any movement of either the transmit or receive telescopes or their respective physical mounting platforms. As a result of such misalignment, the transmitted light beam may not be incident upon the focal plane of the receiving telescope, or may only be partially incident thereupon, leading to a loss or degradation of communications connectivity.
Another problem with free-space optical communications results from the variation in atmospheric conditions. Specifically, since conditions like fog or snow can interfere with the transmitted light beam in such systems, the transmit telescope must produce a light beam with power sufficient to maintain communications connectivity in such variable conditions. In the absence of such signal-degrading conditions, however, the power of the received light beam may overload the electronics of the receive telescope. While the power to the laser or laser amplifier can be reduced to compensate, this may mean operating the devices at gains where they do not operate efficiently.
The aforementioned problems related to initial alignment and to potential loss of communications connectivity due to misalignment occurring during communications operations are essentially eliminated with the present invention.
In accordance with the present invention, during initial alignment of the transmit and receive telescopes, the cross-sectional area of the transmitted light beam is increased beyond the normal cross-sectional area of a focused (parallel) beam at the point where the receive telescope is or should be. The cross-section of the transmitted beam is thus physically larger at the receiving end, thereby increasing the likelihood that the received light beam will be incident upon the focal plane of the receive telescope. This increase in the cross-sectional area of the transmitted beam is effected by varying the divergence of the transmitted beam. By monitoring at the receive telescope a measurable signal parameter such as, for example, received signal power, and providing some type of feedback to the transmit telescope, the divergence of the transmitted light beam can be varied until the beam is incident upon the optical fiber at the focal plane of the receive telescope. Once the transmitted beam is detected at the receive optical fiber by measuring a detectable level of receive signal power, the transmit optical fiber and/or the receive optical fiber are aligned with each other to maximize the received signal power by, in a first embodiment, physically moving the entire transmit telescope until maximum transmitted power is incident upon the receive telescope. Alternatively, in accordance with a second embodiment, instead of moving the entire telescope apparatus to achieve alignment, certain telescope mirror designs, such as those defined by a non-standard conic constant, allow for the movement of only the optical fiber located at the focal plane of the transmit telescope such that the transmitted light beam is incident upon the receive telescope.
In further accordance with the present invention, after the transmitted beam is diverged, the receive telescope is physically moved so that maximum power of the transmitted beam is incident upon the receive optical fiber.
Once alignment is accomplished, by moving the transmit telescope and/or the receive telescope, or by moving the optical fiber at the focal plane of either or both of those telescopes, maximum receive power is achieved by decreasing the divergence of the transmitted light beam.